Through air drying (TAD) has changed the industry's ability to produce soft, bulky, premium quality paper products, particularly in the area of single-ply products. TAD has become the preferred choice for newly purchased paper machines because it can provide improved product attributes and therefore, economic advantages to manufacturers when compared with the products produced by conventional wet pressing (CWP). The advent of TAD has made it possible to produce paper products with good initial softness, bulk and absorbency.
In the older conventional wet pressing method, premium quality paper product, tissues and towels, are normally made by forming a nascent web in a forming structure, transferring the web to a dewatering felt where it is pressed to remove moisture and adhering the web to a Yankee dryer. When the web is dried to a solids content greater than 90%, the web is creped from the Yankee dryer and reeled. This process is referred to as a dry creping process because it occurs after the paper web has been sufficiently dried on the Yankee dryer. Dry creping is discussed in John F. Oliver, Drycreping of tissue paper--a review of basic factors, Yankee Dryer and Drying, A TAPPI PRESS Anthology, pages 215-219, which is herein incorporated by reference.
An alternative to dry creping has been a process called wet creping. In a standard wet creping process, the web is formed in the forming structure and transferred to a press felt where it is pressed to mechanically remove water, just as in the dry creping process. In the wet creping process however, the web is adhered to the Yankee dryer, but creped before the web is considered dry, e.g., at a solids content of less than 90%, generally between 40 to 75%. While wet creped webs exhibit higher levels of absorbency than conventional dry creped webs, they also tend to be stiffer and less soft. Wet creped webs while improving on the absorbency of dry creped webs still do not reach the levels of absorbency achieved by the unpressed TAD webs.
Conventional wet pressing, however, has certain advantages over TAD including 1) lower energy costs associated with the mechanical removal of water rather than drying by the passage of hot air; and 2) increased production speeds. Stated differently, energy consumption is lower and the production speeds can be considerably higher than those used in TAD. Thus, there is a need for processes which can attain the attributes of a TAD web without the concomitant expense associated with an unpressed TAD web.
Many have attempted to improve upon the wet crepe process to achieve the benefits of higher absorbency without the added cost of TAD. We have surprisingly found that the wet crepe process according to the present invention produces improved product attributes in webs that are subsequently dried by any art recognized method, including TAD.
One prior proposed method includes the addition of a molded or sculpted pattern to the wet paper web. U.S. Pat. No. 5,851,353 discloses a drying fabric that is capable of imprinting the web during the drying process. Both U.S. Pat. Nos. 5,505,818 and Re. 28,459 disclose wet creping processes followed by application of the wet web to a TAD impression fabric where the web is dried to completion.
While these final drying techniques can improve desired web properties through wet molding, they can be significantly limited by issues surrounding production speed. Wet crepe webs typically suffer from handling problems, and as disclosed in the '353 patent, a significant portion of the final drying must be done while the web is sandwiched between two fabrics. This causes the drying rate to be reduced to very low levels with water removal rates on the order of 1-3 lbs/hour/ft.sup.2. Conventional rates for can drying are on the order of 10 lbs/hour/ft.sup.2 and conventional TAD removes 10-25 lbs of water/hour/ft.sup.2. Both the '818 patent and the '459 reissue also show very low drying rates. These rates are based upon the difficulty in economically pulling a sufficient volume of heated air through the web.
Others have attempted to offset the disadvantages of wet pressing through improved dry creping processes. U.S. Pat. Nos. 4,448,638 and 4,482,429 teach that superior web properties can be attained by making the adhesion of the web to the dryer greater than its internal cohesion. This was achieved through the use of debonders applied to the web in the wet end of the papermaking process. This technique proves unworkable, however, for the production of towels where strength and absorbency are key attributes since the addition of debonder negatively impacts both.
U.S. Pat. No. 4,992,140 discloses a similar reduction in web bond strength independent of the Yankee adhesion. As disclosed in the '140 patent, an amount of water is applied to the outside of the web just prior to creping thereby increasing the average web consistency from 2 to 10% with a concomitant increase in absorbency on the order of 10-25%. These benefits were, however only seen in higher basis weight webs and at higher evaporative loads in the final drying section, thereby making them less economically preferred.
U.S. Pat. No. 2,995,180 discloses a creping blade having alternating bands that contact the Yankee dryer resulting in a web that was smooth on one side and pearlized on the other. The disclosed pearls, like ridges, opened the web in those areas thereby increasing the caliper of the web. This technology suffers from the disadvantage that it may only be applied to very strong webs since only half the web is creped from the Yankee dryer, while the other half of the web is pulled from the Yankee dryer.
U.S. Pat. Nos. 5,494,554 and 5,730,839 disclose that when adhesion to the Yankee surface is increased during dry creping and the correct blade angle is used, high levels of web breakup can be expected. Each of these processes are however, concerned with a dry creping process, where the web solids content is greater than 95% and the web temperature is on the order of 235.degree. F. Wet creping, by contrast, is carried out a web solids content, typically of, 40 to 75%. U.S. Pat. No. 5,377,428 discloses that in the solids content range where wet creping usually takes place, the web temperature is expected to be about 180.degree. F. to about 200.degree. F.
U.S. Pat. No. 5,336,373 discloses that during the drying process the water that remains in the web migrates to the parts of the web that are in contact with the heated surface. The '373 patent established this through the use of dyes that did not adhere to the fibers during the drying process. With the water migration toward the heated surface, during wet creping, one would expect that the highest water content would be in the Yankee side of the web. Both this high moisture content and the low temperatures would tend to reduce the strength of the adhesive bond between the web and the Yankee dryer, thus, resulting in a reduced creping effect.
U.S. Pat. No. 5,556,511 discloses a non-creped product that is produced on a heated press. The '511 patent teaches that a general explosion of the sheet may be accomplished by the steam within the sheet. This explosion of the sheet is a disruption of the fibers, but is not a clear delamination plane within the sheet that effectively results in plies within the web. Exploded sheets such as those disclosed in the '511 patent suffer large tensile losses, while in a delaminated sheet tensile losses are significantly reduced.
The current trend is away from conventional wet pressing in favor of TAD production since TAD offers the advantages higher caliper, molded or shaped webs and improved absorbency and softness. The present invention utilizes the advantageous cost of conventional wet pressing, but achieves product attributes of bulk, absorbency and softness that are much closer to TAD than anything previously known.
The present invention overcomes the disadvantages associated with prior art wet creping processes and produces a web that parallels that formed through the more expensive TAD processes. The present invention also surprisingly provides a paper web with superior handling properties during the wet creping process.
Further advantages of the invention will be set forth in part in the description which follows and in part will be apparent from the description. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.